1. Field of the Invention
The present invention relates to a method for producing a rotationally symmetrical screening device.
2. Description of the Related Art
Screening devices may be used for example for screening of fibrous stock suspensions, as is done in pressure screens in the paper industry. The fibers that are contained in the suspension are to hereby penetrate through the screen, whereas the undesirable solid components are rejected at the gap and are directed out of the screen device.
Due to the fact that the openings are essentially longitudinal, in other words are slots or gaps, fibrous particles pass easier if both types are present in similar dimensions. With this type of screening technology very effective discharging of non-fibrous contaminants from the fibrous stock suspension is thus made possible. Also conceivable is utilization for separating of different fibrous components—a so-called fiber fractioning. However, a prerequisite herein is high precision of the slot size across the entire screen surface. For stringent requirements, cylindrical screening devices in the embodiment of so-called screen baskets are mostly used.
The screening devices can either be arranged for centrifugal operation with a suspension flow from the inside toward the outside and with profiled bars inserted on the inside edge of the bar holders, or for a centripetal operation with a suspension flow from the outside toward the inside and with profiled bars inserted on the outside edge of the bar holders. With screening devices for centrifugal operation which have bar holders arranged on the outside and profiled bars arranged on the inside, the curvature radius of the bar holders is generally greater when inserting the profiled bars than the radius of curvature of the screen cylinder. Subsequently the radius of curvature of the bar holders is reduced for wedging of the profiled bars. In the case of centripetal operation with bar holders arranged on the inside and profiled bars arranged on the outside the radius curvature of the bar holders during insertion of the profiled bars is generally less than the radius of curvature of the screen cylinder. Subsequently the radius of curvature of the bar holders is increased for wedging of the profiled bars.
To produce screening devices for a centrifugal operation, straight bar holders are generally produced initially and the profiled bars are inserted into these bar holders. The bar holders with the inserted profiled bars are subsequently bent into an open C-shape. Finally, the bar holders are formed into closed rings and their two abutment ends are firmly connected with one another. A method of this type is described for example in DE 39 27 748 A1.
In contrast, to produce screening devices for centripetal operation, curved bar holders consisting of several segments are generally produced initially, whose radius of curvature is less than that of the finished screening device. After the profiled bars are inserted into these bar holders, rings are formed, whereby the radius of curvature of the bar holders is enlarged through reshaping, as a result of which the recesses narrow again, thus wedging the profiled bars. The two abutment ends of the bar holders are subsequently again connected firmly with one another. A relevant method is described for example in DE 10 2006 008 172 A1.
Conventional screening devices for centrifugal operation with merely wedged profiled bars may be usable only to a limited extent, depending upon the respectively used rotor or the respective application. The profiled bars were therefore welded together with the annular bar holders. During operation however failures occurred in a multitude of screening devices within a very short period of time, the reason for which is found in the welding process.
In the case of screening devices designed for centripetal operation where the profiled bars were also welded together with the annular bar holders the same problems occurred as did with the screening devices designed for centrifugal operation. In one initially only wedged version that was arrived at through pre-bending, sufficient wedging was not achieved. Also, breaks occurred with this version. An additional problem with some screening devices results in connection with the attachment of the ferrules of the screening device or respectively screen baskets, regardless of whether the profiled bars are welded together with the bar holders or not. The main stress during operation of the screening device may occur most strongly in the region of the welded seam at the ferrule, since this is also the force transfer point with the greatest tension stress. This too represents a permanent weak point that repeatedly leads to the ferrules breaking off the perforated screen at this location. The reason for this is also based in the welding process.
US 2011/0005 981 A1 describes an alternative cylindrical screening device wherein at least one additional strip-like bar holder with recesses that are open at the edges is inserted in addition to the first strip-like bar holders.
With some prior art methods and screening devices, the described stability problems entail restrictions during use for certain rotors, rotor speeds and design variations.
What is needed in the art is a method as well as a screening device in which the previously discussed problems are eliminated. Such a method is intended to produce screening devices, in particular screen baskets having optimum strength as simply as possible and accordingly cost effectively and without the requirement to weld the profiled bars to the bar holders. With optimum strength and accordingly greater breakage resistance and durability such screening devices are intended in particular for use in more varied applications. Moreover, the risk of breakage in the region of the welding areas between the ferrules and the profiled bar bond is to be minimized as much as possible.